The exam manual is a good place to start for this one:
https://bsaaml.ffiec.gov/manual/RisksAssociatedWithMoneyLaunderingAndTerroristFinancing/14Review the risks and mitigation sections and determine what you need to do to be comfortable that they aren't laundering money, then document it. The first thing examiners usually look at - so the first question you should be confident of - is how they're replenishing the ATM. In your case, do the checks he's cashing equal the ACH credits he's receiving from the ISO, after deducting a reasonable ATM surcharge? These credits represent the ATM withdrawals. If the credits are more than the replenishment checks, they are putting additional cash into the machine, and you need to know that that isn't illicit.
Every bank has it's own risk tolerance, but if the owner is uncooperative about supplying whatever information you request, the customer probably isn't worth keeping. POATMs garner a ton of scrutiny and demand clear due diligence. It may be that they're running the cash through you because their old bank didn't like it. Send them on to the next bank if they're uncooperative.